Noncovalent electrostatic

A different pH-triggered deshielding concept with hydrophilic polymers is based on reversing noncovalent electrostatic bonds [78, 195, 197]. For example, a pH-responsive sulfonamide/PEl system was developed for tumor-specific pDNA delivery [195]. At pH 7.4, the pH-sensitive diblock copolymer, poly(methacryloyl sulfadimethoxine) (PSD)-hZocA -PEG (PSD-b-PEG), binds to DNA/PEI polyplexes and shields against cell interaction. At pH 6.6 (such as in a hypoxic extracellular tumor environment or in endosomes), PSD-b-PEG becomes uncharged due to sulfonamide protonation and detaches from the nanoparticles, permitting PEI to interact with cells. In this fashion PSD-b-PEG is able to discern the small difference in pH between normal and tumor tissues. 

We review the subject of noncovalent interactions in proteins with particular emphasis on the so-called weakly polar interactions. First, the physical bases of the noncovalent electrostatic interactions that stabilize protein structure are discussed. Second, the four types of weakly polar interactions that have been shown to occur in proteins are described with reference to some biologically significant examples of protein structure stabilization and protein-ligand binding. Third, hydrophobic effects in proteins are discussed. Fourth, an hypothesis regarding the biological importance of the weakly polar interaction is advanced. Finally, we propose adoption of a systematic classification of electrostatic interactions in proteins. 

Dipole-dipole force (Section 2.13) A noncovalent electrostatic interaction behveen dipolar molecules. 

Zn 208,209 noncovalent electrostatic into actions (charge-induced dipole... 

The native state of a single-domain protein is its tertiary structure, and it is estimated that approximately 1000 different shapes or folds exist in nature. The native state generally adopts a globular three-dimensional shape, stabilized by both covalent (peptide bonds, disulfide bridges) and noncovalent (electrostatic, hydrophobic and hydrogen bonds) interactions. Local structures within the native fold are known as secondary structures, with common motifs including alpha heHces and beta sheets. The classic experiments of Anfinsen et al. su ested a Thermodynamic Hypothesis for folding, in which the... 

Several of these topological types (i.e., linear, cyclic, clusters) are remarkably reminiscent of those that are obtained by noncovalent self-assembly of proteins [8]. This offers a glimpse of the possible covalent topologies however, virtually no examples of dendrimer self-assembly by noncovalent, electrostatic type interactions have been reported until recently. Such abiotic, noncovalent PAMAM (G = 9) dendrimer self-assemblies have now been observed by atomic force microscopy (AFM) on freshly cleaved mica surfaces [268] (Figure 42). 

However, virtually no reports have appeared concerning the self-assembly of dendrimers by noncovalent electrostatic methods until recently. Aida et al. [191] have reported the electrostatically directed assembly of prophyrin core dendrimers to produce large infinite aggregates as illustrated in Fig. 38. Energy transfer constants were obtained and used to determine distances between acceptors and donors as it was demonstrated that communication occurred between those domains. 

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